1. Field of the Invention
The present invention generally relates to a constant voltage/constant frequency power supply apparatus under control of pulse-width modulation. More specifically, the present invention is directed to a PWM (pulse-width modulation)-control type uninterruptive power source (UPS) whose ground potential has no high-frequency (modulaton frequency) signal components.
2. Description of the Related Art
In FIG. 1, there is shown a major circuit of a typical constant voltage/constant frequency power supply apparatus known as an uninterruptive power source in the field.
A circuit arrangement of this power supply apparatus is as follows: A voltage of a single-phase AC power supply source 1, one line of which is grounded, is rectified in a full waveform rectifying mode by a bridge rectifier 2 constructed of diodes 21 to 24. The rectified voltage is converted into a predetermined DC voltage via a step-up chopper 7 constructed of a DC reactor 71, a switching element 72, and a diode 73, and a smoothing capacitor 3. Thereafter, the resultant DC voltage is again converted into a corresponding AC voltage by way of pulse-width-modulation (PWM) controlling operations by an inverter circuit 4 arranged by a bridge-construction of switching elements 41 to 44. The resultant AC voltage is filtered by an L-type filter 5 including a reactor 51 and a capacitor 52 so as to eliminate high-frequency signal components and converted into a smoothed sinusoidal wave. Finally, this sinusoidal wave voltage is applied to a load 6.
The function of the step-up chopper 7 is to increase the input voltage in order that the output AC voltage is equal to the power source voltage. Alternatively, the input AC voltage may be stepped up by, for instance, a transformer and thereafter may be rectified.
In fact, according to the above-described conventional power supply apparatus, the high-frequency signal components have been eliminated from the AC voltage applied to the load. However, the high-frequency signal components appear in the ground potentials due to an employment of the PWM control operations.
Referring now to waveforms shown in FIG. 2, a description will be made why the high-frequency signal components are contained in the ground potentials.
Assuming that a supply voltage (a voltage across L-N terminals) is equal to "V.sub.1 ", a ground potential "v.sub.N " at a ground side "N" of the power source 1 is equal to zero ("v.sub.N "=0), whereas another ground potential "v.sub.L " at a non-ground side "L" thereof is equal to "V.sub.1 " (v.sub.L =V.sub.1) (see FIG. 2A). A ground potential "v.sub.DN " at a load side "DN" of a DC output from the rectifier 2 is determined by conducting the diodes 22 and 24. That is, during the positive period of the power source voltage V.sub.1, the ground potential v.sub.DN is equal to zero (v.sub.DN =0) since one diode 24 is turned ON. During the negative period of the power source voltage V.sub.1, the ground potential v.sub.DN is equal to V.sub.1 (v.sub.DN =V.sub.1). As a result, if the DC voltage is "E.sub.o ", another potential voltage v.sub. DP at the positive side "DP" is determined by: EQU v.sub.DP =v.sub.DN +E.sub.o,
where E.sub.o is nearly equal to a constant (see FIG. 2B).
A ground potential "v.sub.V " at an AC output "V" phase of the inverter circuit 4 is determined by turning ON/OFF the switching elements 43 and 44. When one switching element 43 is turned ON, the ground potential V.sub.v is equal to v.sub.DP (v.sub.V =v.sub.DP), whereas when the other switching element 44 is turned ON, the ground potential v.sub.V is equal to v.sub.DN (v.sub.V =v.sub.DN). In general, all of these switching elements are turned ON/OFF at a high speed (e.g., 10 to 20 KH.sub.Z) under PWM control so that the ground potential of the V phase "v.sub.V " contains the high-frequency signal (noise) components as "v.sub.DP " and "v.sub.DN " being envelope lines because of the PWM control operation (see FIG. 2C).
Another ground potential "v.sub.U " at U phase of the AC output is determined by the following equation, if the AC output voltage is equal to "V.sub.o ": EQU v.sub.U =v.sub.V +V.sub.o.
As a consequence, in case that the AC output voltage V.sub.o is equal to the power source voltage V.sub.1 and also has a in-phase condition thereto, the ground potential "v.sub.U " is represented by a waveform shown in FIG. 2D. This ground potential "v.sub.U " contains the high-frequency signal components similar to the above-described ground potential "v.sub.V ". A maximum value "v.sub.U (max)" of this ground potential "v.sub.U " is defined by: EQU v.sub.U (max)=a peak value of V.sub.o +E.sub.o.
It is obvious that this maximum value "v.sub.U (max)" is considerably higher than the AC output voltage V.sub.o from the inverter 4.
In case that the AC output voltage V.sub.o is equal to the power source voltage V.sub.1 and also has a reverse-phase condition thereto, the ground potential "v.sub.U " is represented by a waveform shown in FIG. 2E. This ground potential "v.sub.V " also contains the high-frequency signal components.
As described above, according to the conventional PWM controlled power supply apparatus, since the variations in the ground potentials at the output terminal thereof contain the high-frequency signal (noise) components produced by the high speed switching operation of the DC/AC inverter, it is necessary to employ a large-scale line filter so as to filter out such high-frequency noise components. In particular, as a power, supply apparatus used for computers, the high-frequency noises must be completely eliminated. Also, if a surge suppressor capable of absorbing indirect lightning or switching surges is provided between the line and ground, this surge suppressor may be burned out. Specifically, since the very high voltage is instantaneously applied, the rated voltage of the surge suppressor must be selected to be a proper high value.